The formal way to radius the end of a connecting rod is to use a rotary table and an end mill. If you don’t have a rotary table you can make hardened filing buttons and use these to file a radius on the end of the rod.
The way I want to describe here uses the hole through the end of the rod to create a rotation point. The hole needs to be drilled precisely and carefully, some hints and tips for dripping holes to size are written in a separate article: Accurately Drilling a Hole
The drill or a shaft of the correct diameter is placed through the hole in the beam and this is used as the rotation point, this sets the height of the beam above the top surface of the machine vice.
Now use a slot drill or a fly-cutter set to a height that aligns with the upper surface of the beam when it is horizontal in the vice.
From here onwards do not change the height of the cutting tool. This is the height that will be used for all of the cuts.
The process is simple: slacken the vice slightly, rotate the beam a small amount in the vice ensuring that the drill/axle remains in contact with the top surface of the vice. Make a cut across the surface of the beam.
We’ve now got halfway and if the beam is longer than the height of the vice jaws it will be necessary to turn the beam over and to start the process of machining the other side.
We’ve now machined a complete radius on the beam. There are steps or flats in the machined parts and this will depend on the number of cuts you’ve made.
The final process is to file the radius gently removing the flats.
The only problem with this process is if the beam is much longer than the height of the vice jaws.
As you can see, this means you can only complete part of the machining and you will need to finish the task by filing the part.